Abstract

Carrier-envelope-offset (CEO) and pulse-repetition frequencies of a Ti:sapphire-pumped femtosecond optical parametric oscillator were locked to uncertainties of 0.09 Hz and 0.16 mHz respectively, with the CEO beat signal linewidth being stabilized to 15 Hz (instrument limited). In-loop phase-noise power spectral density measurements showed a contribution of our servo electronics to the comb-line frequency uncertainty of up to 110 Hz. Complementary time-series data implied an in-loop comb instability of 2 x 10−11 (1-s gate time), matching the Rb-stabilized reference used and verifying that dual servo-control of the CEO and repetition frequencies was effective in stabilizing the comb to at least this precision.

In-loop measurement (1 s observation time) of the phase-noise PSD for (a) fCEO and (b) frep. The integrated phase-noise from 1 Hz - 2 MHz is 0.56 rad (0.98 mrad) for fCEO (frep). (b) The PSD phase-noise for frep is shown for the 8th harmonic, and the integrated data are scaled appropriately to show the cumulative phase-noise in frep.

Frequency fluctuations from a mean value recorded on a frequency counter with a 1 s gate time. (a) In-loop measurement of the fCEO beat showing fluctuations (ΔfCEO) about the 10 MHz frequency lock. (b) In-loop measurement of the frep showing fluctuations about the 280.18 MHz frequency lock. The frep was combined in a mixer with a synthesized signal offset by 20 kHz so the fluctuations shown are for this 20 kHz mixer output signal.

Fractional comb instability (scaled by fn = 200 THz and n = 714285) from in-loop frep and fCEO signals. Data for the electronic system (Δfelec/felec) and the Rb clock (ΔfClock/fClock) show the intrinsic uncertainty in the frep measurements.